Satisfying concrete’s thirst with water-reducing admixtures

Responsible contractors have realised the growing strain on South Africa’s scarce water resources, and CHRYSO is supporting their conservation efforts with its water-reducing admixtures for concrete.

A recent example of this was in the construction of the Vlakfontein Reservoir near Standerton in the province of Mpumalanga. This is probably one of the largest cylindrical post-tensioned concrete reservoirs in the world, with an inner diameter of 154 metres and a heavily reinforced 11,8 metre high wall.

Better slump with less water

According to Patrick Flannigan, technical manager : concrete business unit at CHRYSO Southern Africa, the project made use of several CHRYSO® admixtures to reduce water and improve workability.

“One of these is CHRYSO® Plast Omega 178 plasticiser which, when used in a concrete mix, permits a reduction in water content without affecting the consistency, also referred to as the workability,” says Flannigan. “It can also increase the slump – or flow – without affecting the water content of the mix.”

The Vlakfontein Reservoir has a ground water drainage system constructed with three layers of 5 MPa no-fines concrete. The 8 000 m³ of no-fines concrete comprises a 50 mm thick layer of 9,5 mm stone mix, followed by a 245 mm and 125 mm layer of 19 mm aggregate.

CHRYSO® Easydrain was introduced to these mixes to facilitate consistency and ensure the aggregate would bind mechanically and hydraulically to the cement paste. This specialised admixture fluidises the concrete, assists with a homogenous mix and improves curing to reduce the chance of cracking. The bonded aggregates, once dried, will not loosen. An additional drainage layer of around 2 200 m³ of no-fines concrete will be poured around the reservoir.

Shrinking reducing admixture

Once the no-fines concrete was completed, a layer of 200 mm 35 MPa concrete was pumped in 13 pours of approximately 250 m3 each. This mix included CHRYSO® Serenis, a shrinkage reducing admixture. This, together with CHRYSO® Omega 178 superplasticiser, reduces water and assists with workability.

CHRYSO® CWA 10 crystalline waterproofing agent will provide integral waterproofing and reduce permeability in the concrete. The crystals prevent water penetration and protect the concrete against corrosive ground water and chemicals.

CHRYSO’s water-reducing admixtures were also used in the Khutsong Reservoir near Carletonville in Gauteng. Due to the danger of sinkholes in this region, and the possibility of ground movement, the reservoir is the first to be constructed with a pre-tensioned concrete floor.

The scale of the work required 212 truckloads of concrete for the floor, successfully poured within a day. The 1,267 m³ concrete floor used pumped mixes at 35 MPa at a thickness of 450 mm. In this demanding contract, the concrete mix included CHRYSO® ZA 1559, a new generation, multi-dose water-reducing plasticiser.

“This admixture allows a wide range of dosages to be applied, without any excessive retardation at the higher dosages,” he says. “The multi-dose characteristic of CHRYSO® ZA 1559 also gives concrete extended workability characteristics.”

Flannigan notes that it improves the cohesion and lowers the viscosity of a concrete mix. This results in an improved homogeneity and compaction, allowing for superior off-shutter finishes. With less need to add extra water, CHRYSO® ZA 1559 reduces the permeability of concrete, thereby improving is durability.

“The product may be used in mixes extended with limestone or commonly used supplementary cementing materials (SCMs) like ground granulated blast furnace slag (GGBS), fly ash and silica fume,” he says. “At the same time, it does not undermine the early age strength of concrete. In fact, in certain cases, it may be used to improve it.”

Readymix admixtures meeting the standard

CHRYSO® ZA 1559 has applications in readymix concrete and mechanically mixed site concrete, as well as conventionally placed, pumped and highly reinforced concrete. It conforms to the requirements of SANS 50934-2 (EN 934-2) standards, which are approximate equivalents of ASTM C494 Type A.

Water evaporation was successfully controlled with CHRYSO® Profilm 19 evaporation reducer, while shrinkage of the reservoir floor was reduced using CHRYSO® Serenis shrinkage reducing agent. A minimum of 180 litres of water per m3 had to be used and fly ash was added as the heat of hydration had to be controlled due to the mass pour. The end strength did not exceed 45 MPa.